I’ve been receiving a few questions from the readers. (Yes, I actually have readers—other than you.)
I misread the email; the initial version of the I/Q noise figure discussion was completely wrong.
I/Q Noise Figure
What’s the difference of noise figure between quadrature [downconverter] and single downconverter? In the quadrature downconverter, you have I and Q path, if required Nf=10dB for this quadrature downconverter, what’s the Nf for the path of I or Q? Can I see the Nf for RF to I path is 7dB, the same thing for Q?
— K. C.
Unfortunately, the answer is no. The important thing to remember when computing things in dB (and summing them) is whether they add in-phase (constructively) or not. Since the I and Q signals are completely orthogonal, if a 10 dB NF is required for the quadrature downconverter (I + jQ), then 10 dB is required for the each of the I and Q paths.
If, however, we had two paths that summed constructively, and we needed a 10 dB overall noise figure, then yes, each path would only need to deliver 10 dB (per path). Unfortunately, I and Q don’t do this constructive summation.
Loop gain in Cadence ADE
Can u please elaborate on loop gain/Stablity simulation using Cadence ADE?
— H. M.
Unfortunately, the answer is no. Or sort of. Sure, I can elaborate, but I can’t do a very good step-by-step job because I don’t have access to Cadence ADE. So, I’m running on my own memory, which (as my friends know) is very dangerous.
However, here’s a rough step-by-step:
- Break the loop by inserting an Iprobe element. (Usually, I choose an Iprobe). You usually want to break the loop in a point that’s high-impedance. Typically, I choose the highest impedance output node (transconductor) looking into a compensation capacitor. So, the Iprobe would measure current going from the transconductor into its compensation capacitor.
- Start ADE (Tools –> Analog Environment). If it’s not already set in the ADE window, select Setup (?) –> Simulator/Directory/Host and select spectre.
- Select Setup –> Analysis and select stb (stability). You’ll need to select start and stop frequencies. There’s also a button to select the loop gain element (Iprobe) that you’re using to break the loop. (You can have multiple Iprobes in a single schematic, for each loop you want to analyze.)
- After you run it, you can choose Outputs –> Plot –> Loop Gain (?) or something like that to see amplitude and phase vs frequency. You can also choose the loopgain result from the results browser and do a direct phaseMargin() measurement on it, which will pick the phase margin. (Something like phaseMargin(-getData(‘loopgain’ ?result ‘stb’)) – but I’m typing from memory.)
- Keep in mind that Cadence returns the exact loop gain, which is (or should be) 180 degrees at dc. However, the phaseMargin() function expects the loop gain to be 0 degrees at dc and then go to 180 (unstable) at some point later. So, you have to invert the result when using phaseMargin().
The stability analysis, I believe, uses the Middlebrook method,which really runs two ac analyses and combines the results to produce the return ratio. Most other circuit simulators (HSPICE RF) have a similar capability. If your simulator doesn’t, it is possible to get the equivalent by manually running the two analyses.